From be9137a5c5eba6fde096232e6fe908ab0da2579e Mon Sep 17 00:00:00 2001
From: Michael Ilseman <milseman@apple.com>
Date: Tue, 27 Nov 2012 00:46:26 +0000
Subject: [PATCH] Fast-math optimization: fold multiply by zero

Added in first optimization using fast-math flags to serve as an example for following optimizations. SimplifyInstruction will now try to optimize an fmul observing its FastMathFlags to see if it can fold multiply by zero when 'nnan' and 'nsz' flags are set.

llvm-svn: 168648
---
 .../llvm/Analysis/InstructionSimplify.h       |  9 +++++
 llvm/lib/Analysis/InstructionSimplify.cpp     | 39 +++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/llvm/include/llvm/Analysis/InstructionSimplify.h b/llvm/include/llvm/Analysis/InstructionSimplify.h
index 6db400c563d6..ce7818184e9a 100644
--- a/llvm/include/llvm/Analysis/InstructionSimplify.h
+++ b/llvm/include/llvm/Analysis/InstructionSimplify.h
@@ -25,6 +25,7 @@ namespace llvm {
   class DominatorTree;
   class Instruction;
   class DataLayout;
+  struct FastMathFlags;
   class TargetLibraryInfo;
   class Type;
   class Value;
@@ -43,6 +44,14 @@ namespace llvm {
                          const TargetLibraryInfo *TLI = 0,
                          const DominatorTree *DT = 0);
 
+  /// Given operands for an FMul, see if we can fold the result.  If not, this
+  /// returns null.
+  Value *SimplifyFMulInst(Value *LHS, Value *RHS,
+                          FastMathFlags FMF,
+                          const DataLayout *TD = 0,
+                          const TargetLibraryInfo *TLI = 0,
+                          const DominatorTree *DT = 0);
+
   /// SimplifyMulInst - Given operands for a Mul, see if we can
   /// fold the result.  If not, this returns null.
   Value *SimplifyMulInst(Value *LHS, Value *RHS, const DataLayout *TD = 0,
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index a76e5ad1b8f8..e3fe9240d0fd 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -886,6 +886,33 @@ Value *llvm::SimplifySubInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
                            RecursionLimit);
 }
 
+/// Given the operands for an FMul, see if we can fold the result
+static Value *SimplifyFMulInst(Value *Op0, Value *Op1,
+                               FastMathFlags FMF,
+                               const Query &Q,
+                               unsigned MaxRecurse) {
+ if (Constant *CLHS = dyn_cast<Constant>(Op0)) {
+    if (Constant *CRHS = dyn_cast<Constant>(Op1)) {
+      Constant *Ops[] = { CLHS, CRHS };
+      return ConstantFoldInstOperands(Instruction::FMul, CLHS->getType(),
+                                      Ops, Q.TD, Q.TLI);
+    }
+ }
+
+ // Check for some fast-math optimizations
+ if (FMF.NoNaNs) {
+   if (FMF.NoSignedZeros) {
+     // fmul N S 0, x ==> 0
+     if (match(Op0, m_Zero()))
+       return Op0;
+     if (match(Op1, m_Zero()))
+       return Op1;
+   }
+ }
+
+ return 0;
+}
+
 /// SimplifyMulInst - Given operands for a Mul, see if we can
 /// fold the result.  If not, this returns null.
 static Value *SimplifyMulInst(Value *Op0, Value *Op1, const Query &Q,
@@ -951,6 +978,14 @@ static Value *SimplifyMulInst(Value *Op0, Value *Op1, const Query &Q,
   return 0;
 }
 
+Value *llvm::SimplifyFMulInst(Value *Op0, Value *Op1,
+                              FastMathFlags FMF,
+                              const DataLayout *TD,
+                              const TargetLibraryInfo *TLI,
+                              const DominatorTree *DT) {
+  return ::SimplifyFMulInst(Op0, Op1, FMF, Query (TD, TLI, DT), RecursionLimit);
+}
+
 Value *llvm::SimplifyMulInst(Value *Op0, Value *Op1, const DataLayout *TD,
                              const TargetLibraryInfo *TLI,
                              const DominatorTree *DT) {
@@ -2799,6 +2834,10 @@ Value *llvm::SimplifyInstruction(Instruction *I, const DataLayout *TD,
                              cast<BinaryOperator>(I)->hasNoUnsignedWrap(),
                              TD, TLI, DT);
     break;
+  case Instruction::FMul:
+    Result = SimplifyFMulInst(I->getOperand(0), I->getOperand(1),
+                              I->getFastMathFlags(), TD, TLI, DT);
+    break;
   case Instruction::Mul:
     Result = SimplifyMulInst(I->getOperand(0), I->getOperand(1), TD, TLI, DT);
     break;
-- 
2.34.1